1. Technical Field
This disclosure is generally related to reduction of flicker perception in solid state lighting systems.
2. Description of the Related Art
Solid state lighting has many advantages over traditional lighting, such as incandescent and fluorescent bulbs. Solid state lighting devices exhibit lower energy consumption, longer lifetime, and improved robustness over traditional lighting. However, the existing approaches to electrically drive solid state lighting are known to generate undesirable side effects.
In particular, existing approaches may generate electromagnetic interference (EMI) and switching noise. Existing approaches generally employ a configuration of a switch-mode power supply to convert alternating current (AC) power to direct current (DC) power to operate the solid state light sources. Switch-mode power supplies excite a transformer by rapidly operating a switch, such as a metal-oxide semiconductor field effect transistor (MOSFET). The excitation of the transformer generates EMI that may interfere with signals intended for consumer electronics, such as cell phones, televisions, and radios. Additionally or alternatively, the rapid operation of the switch generates switching noise that may couple back to the main power line. Thus, using a switch-mode power supply to drive solid state lighting may generate the undesirable side effects of EMI and switching noise.
As an alternative to employing a switch-mode power supply, a solid state light source may be connected to directly to an alternating current (AC) line voltage or to a rectified AC line voltage. However, such operation of a solid state light source may result in visually, physiologically, and/or cognitively perceptible flicker. Flicker is a lighting performance factor similar to color rendering, color temperature, and glare and may affect user satisfaction and technology adoption. Lighting industry standards groups are still developing a unified definition of flicker, so a unified industrial standard definition does not currently exist. However, flicker may be defined as a periodic change in the instantaneous light output of a light source. More particularly, flicker may be defined as a periodic change in the instantaneous light output of a light source that may be visually, physiologically, and/or cognitively perceived. Because the brightness of the output of a solid state light source is a function current, the drive current may be used to characterize some aspects of flicker. For example, frequency of the flicker may be defined by the occurrences of cycles of the periodic change in light output or drive current per unit time, e.g., seconds. Accordingly, solid state light sources may have a flicker frequency that matches the frequency of an AC line current (e.g., mains or grid) or that matches the frequency of a rectified AC line current.
The intensity or amount of the flicker may be defined by or represented as a percent flicker or as a flicker index. The percent flicker is 100 multiplied by the maximum light output (A) in a period minus the minimum light output (B) in a period divided by the sum of the maximum and minimum, i.e., 100*(A−B)/(A+B). The flicker index depends upon an area under a graphical curve of a period of light output that is above an average light output (area 1) and the area under the curve that is below the average light output (area 2). The flicker index is defined by dividing area 1 by the sum of area 1 and area 2, i.e., area 1/(area 1+area 2).
Some government agencies have restricted the use of solid state lighting based on the frequency of flicker and not necessarily on the intensity of the flicker. In particular, the government agencies restrict the use of solid state lighting that exhibits flicker frequencies occurring at 120 hertz or slower. Light flicker at low frequencies (e.g., less than 120 hertz) has been attributed to inducing seizures, causing migraine headaches, dizziness, nausea, impaired visual performance, and general malaise. Thus, light flicker occurring at less than about 120 hertz may be visually, physiologically, and/or cognitively perceived through physical health side effects. Until recently, it was believed that light flicker at frequencies up to about 160 hertz was also unhealthy.
Perceptible flicker may become a problem for the LED industry because flicker may contribute to a negative perception of solid state lighting technology as a whole. If most LEDs exhibit flicker, the public may decide that all LEDs flicker. Such may discourage the adoption of LED lighting and/or may spur regulatory restrictions on the use of solid state lighting. Thus, EMI, switching noise, and visual, physiological, and/or cognitive side effects may also hinder the growth of solid state lighting, especially solid state lighting that is excited directly by AC power sources.